1. Field of Invention
The present invention relates to an image forming apparatus based on an electrophotographic system, an electrostatic recording system or the like system.
2. Description of Related Art
In general, an image forming apparatus as an electrophotographic system (printer, copying machine, facsimile or the like) is provided with a fixing unit for applying heat and pressure to a sheet to fix a toner image transferred to the sheet. This fixing unit includes a heating unit for heating and melting toner on a sheet and a pressing unit for pressing the sheet against the heating unit.
The pressing unit of the fixing unit consists, for example, of a fixing roller and a pressure roller which is urged against the fixing roller with a predetermined load. A nip portion is formed between the fixing roller and the pressure roller which is directly or indirectly urged against the fixing roller to hold and convey a sheet therebetween.
The heating unit of the fixing unit consists of a heat source (for example, halogen heater) contained in the pressure roller, and an endless fixing belt which is wound around the fixing roller (heating belt type). In this case, a nip portion is formed by urging the pressure roller against the fixing roller through the fixing belt. Alternatively, the fixing roller may incorporate a heat source, and serves itself as a heating unit (heating roller type). In this case, the pressure roller is urged directly against the fixing roller while a nip portion is formed therebetween.
The image forming apparatus having such a fixing unit develops toner images on photoreceptor drums in correspondence with image data, and transfers the toner images on a sheet. The sheet with the transferred toner images is conveyed to the fixing unit, and passed through the nip portion to fix the toner images with heat and pressure.
This kind of fixing unit is described, for example, in Japanese Patent Published Application No. 06-250560, Japanese Patent Published Application No. 10-221999, and Japanese Patent Published Application No. 09-138598.
In the case of the fixing units described in Japanese Patent Published Application No. 06-250560 and Japanese Patent Published Application No. 10-221999, a nip portion is formed with a fixing roller on which is partly wound an endless belt running around a plurality of rollers. The fixing unit includes a pressure roller located in contact with the fixing roller through the endless belt from the inside of the endless belt at the exit of the nip portion. The fixing unit prevents displacement of images by exerting a braking force on the endless belt conveyed on the pressure roller in order to remove the difference in the conveyance speed between the pressure roller and the fixing roller. On the other hand, in the case of the fixing unit described in Japanese Patent Published Application No. 09-138598, a heat-resistant belt is supported by a plurality of rollers around which this belt is wound. This fixing unit includes a pressure roller urged in contact with a plurality of rollers through the heat-resistant belt. A tension is given to the heat-resistant belt by controlling the plurality of rollers.
When fixing toner in the fixing unit, the surface of a sheet bearing an unfixed toner image comes in direct contact with a heating unit (fixing belt or fixing roller). Accordingly, a latent image may be formed on the heating unit with wax, which is soaked from toner and attached to the heating unit (fixing belt or fixing roller), and may appear on the next image. More specifically, when fixing toner to form the next image, the wax attached to the heating unit appears as the unevenness of gloss (referred to as a gloss memory) corresponding to the unevenness of the attached wax amount.
There is a demand to clear such a gloss memory when fixing toner in a fixing unit and improve the image quality. However, such a gloss memory cannot be prevented from occurring in the fixing units described in Japanese Patent Published Application No. 06-250560, Japanese Patent Published Application No. 10-221999, and Japanese Patent Published Application No. 09-138598.